40,811 research outputs found
Quantum logic for control and manipulation of molecular ions using a frequency comb
Due to their rich level structure, molecules are well-suited for probing time
variation of fundamental constants, precisely measuring parity violation and
time-reversal non-invariance effects, studying quantum mechanical aspects of
chemical reactions, and implementing scalable quantum information processing
architectures. Molecular ions are particularly attractive for these
applications due to their long storage times and the near-perfect isolation
from environment that result in long coherence times required to achieve high
measurement precision and reduce systematic errors. However, the control of
molecular quantum states remains a challenge. Based on quantum logic
techniques, we propose a scheme for preparation, manipulation, and detection of
quantum states of single molecular ions. The scheme relies on coherent coupling
between internal and motional degrees of freedom of the molecular ion via a
frequency comb laser field, while detection and cooling of the motion of ions
is done via a co-trapped atomic ion.Comment: 5 pages, 3 figure
Quantum state preparation and control of single molecular ions
Preparing molecules at rest and in a highly pure quantum state is a long
standing dream in chemistry and physics, so far achieved only for a select set
of molecules in dedicated experimental setups. Here, a quantum-limited
combination of mass spectrometry and Raman spectroscopy is proposed that should
be applicable to a wide range of molecular ions. Excitation of electrons in the
molecule followed by uncontrolled decay and branching into several lower energy
states is avoided. Instead, the molecule is always connected to rotational
states within the electronic and vibrational ground-state manifold, while a
co-trapped atomic ion provides efficient entropy removal and allows for
extraction of information on the molecule. The outlined techniques might enable
preparation, manipulation and measurement of a large multitude of molecular ion
species with the same instrument, with applications including, but not limited
to, precise determination of molecular properties and fundamental tests of
physics.Comment: 12 pages, 2 figures, reformatted for resubmissio
Reciprocatory magnetic reconnection in a coronal bright point
Coronal bright points (CBPs) are small-scale and long-duration brightenings
in the lower solar corona. They are often explained in terms of magnetic
reconnection. We aim to study the sub-structures of a CBP and clarify the
relationship among the brightenings of different patches inside the CBP. The
event was observed by the X-ray Telescope (XRT) aboard the Hinode spacecraft on
2009 August 2223. The CBP showed repetitive brightenings (or CBP flashes).
During each of the two successive CBP flashes, i.e., weak and strong flashes
which are separated by 2 hr, the XRT images revealed that the CBP was
composed of two chambers, i.e., patches A and B. During the weak flash, patch A
brightened first, and patch B brightened 2 min later. During the
transition, the right leg of a large-scale coronal loop drifted from the right
side of the CBP to the left side. During the strong flash, patch B brightened
first, and patch A brightened 2 min later. During the transition, the
right leg of the large-scale coronal loop drifted from the left side of the CBP
to the right side. In each flash, the rapid change of the connectivity of the
large-scale coronal loop is strongly suggestive of the interchange
reconnection. For the first time we found reciprocatory reconnection in the
CBP, i.e., reconnected loops in the outflow region of the first reconnection
process serve as the inflow of the second reconnection process.Comment: 13 pages, 8 figure
A blind deconvolution approach to recover effective connectivity brain networks from resting state fMRI data
A great improvement to the insight on brain function that we can get from
fMRI data can come from effective connectivity analysis, in which the flow of
information between even remote brain regions is inferred by the parameters of
a predictive dynamical model. As opposed to biologically inspired models, some
techniques as Granger causality (GC) are purely data-driven and rely on
statistical prediction and temporal precedence. While powerful and widely
applicable, this approach could suffer from two main limitations when applied
to BOLD fMRI data: confounding effect of hemodynamic response function (HRF)
and conditioning to a large number of variables in presence of short time
series. For task-related fMRI, neural population dynamics can be captured by
modeling signal dynamics with explicit exogenous inputs; for resting-state fMRI
on the other hand, the absence of explicit inputs makes this task more
difficult, unless relying on some specific prior physiological hypothesis. In
order to overcome these issues and to allow a more general approach, here we
present a simple and novel blind-deconvolution technique for BOLD-fMRI signal.
Coming to the second limitation, a fully multivariate conditioning with short
and noisy data leads to computational problems due to overfitting. Furthermore,
conceptual issues arise in presence of redundancy. We thus apply partial
conditioning to a limited subset of variables in the framework of information
theory, as recently proposed. Mixing these two improvements we compare the
differences between BOLD and deconvolved BOLD level effective networks and draw
some conclusions
Three realizations of quantum affine algebra
In this article we establish explicit isomorphisms between three realizations
of quantum twisted affine algebra : the Drinfeld ("current")
realization, the Chevalley realization and the so-called realization,
investigated by Faddeev, Reshetikhin and Takhtajan.Comment: 15 page
On design of robust fault detection filter in finite-frequency domain with regional pole assignment
This brief is concerned with the fault detection (FD) filter design problem for an uncertain linear discrete-time system in the finite-frequency domain with regional pole assignment. An optimized FD filter is designed such that: 1) the FD dynamics is quadratically D-stable; 2) the effect from the exogenous disturbance on the residual is attenuated with respect to a minimized H∞-norm; and 3) the sensitivity of the residual to the fault is enhanced by means of a maximized H--norm. With the aid of the generalized Kalman-Yakubovich-Popov lemma, the mixed H--/H∞ performance and the D-stability requirement are guaranteed by solving a convex optimization problem. An iterative algorithm for designing the desired FD filter is proposed by evaluating the threshold on the generated residual function. A simulation result is exploited to illustrate the effectiveness of the proposed design technique.This work was supported in part by the Deanship of Scientific Research (DSR) at King Abdulaziz University in Saudi Arabia under Grant 16-135- 35-HiCi, the National Natural Science Foundation of China under Grants
61134009 and 61203139, the Royal Society of the U.K., and the Alexander von Humboldt Foundation of Germany
NMR Determination of an Incommensurate Helical Antiferromagnetic Structure in EuCo2As2
We report Eu, As and Co nuclear magnetic resonance
(NMR) results on EuCoAs single crystal. Observations of Eu and
As NMR spectra in zero magnetic field at 4.3 K below an
antiferromagnetic (AFM) ordering temperature = 45 K and its
external magnetic field dependence clearly evidence an incommensurate helical
AFM structure in EuCoAs. Furthermore, based on Co NMR data in
both the paramagnetic and the incommensurate AFM states, we have determined the
model-independent value of the AFM propagation vector = (0, 0, 0.73
0.07)2/ where is the lattice parameter. Thus the
incommensurate helical AFM state was characterized by only NMR data with
model-independent analyses, showing NMR to be a unique tool for determination
of the spin structure in incommensurate helical AFMs.Comment: 6 pages, 4 figures, accepted for publication in Phys.Rev.
Molecular Dynamics Study of Bamboo-like Carbon Nanotube Nucleation
MD simulations based on an empirical potential energy surface were used to
study the nucleation of bamboo-like carbon nanotubes (BCNTs). The simulations
reveal that inner walls of the bamboo structure start to nucleate at the
junction between the outer nanotube wall and the catalyst particle. In
agreement with experimental results, the simulations show that BCNTs nucleate
at higher dissolved carbon concentrations (i.e., feedstock pressures) than
those where non-bamboolike carbon nanotubes are nucleated
Chaotic to ordered state transition of cathode-sheath instabilities in DC glow discharge plasmas
Transition from chaotic to ordered state has been observed during the initial
stage of a discharge in a cylindrical dc glow discharge plasma. Initially it
shows a chaotic behavior but increasing the discharge voltage changes the
characteristics of the discharge glow and shows a period substraction of order
7 period 5 period 3 period 1 period i.e. the system goes to
single mode through odd cycle subtraction. On further increasing the discharge
voltage, the system goes through period doubling, like 1 period 2 period
4 period. On further increasing the voltage, the system goes to stable
state without having any oscillations.Comment: chathode-sheath, instabilities, chaos, period-subtraction,
bifurcation, dc-discharg
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